The possibility for analyzing gas mixtures is an essential boundary condition in the monitoring of industrial plants for an incident-free and safe operation, especially against the background of the increasing degree of automation. To make it possible to recognize gases escaping in case of an incident as accurately and rapidly as possible, for example, in major industrial plants, in petrochemical plants or on drilling platforms, it is desirable to detect the discharge of gases that are hazardous for health even at low concentrations with a high measuring sensitivity directly at the sites of possible incidents.
Optical gas detector systems are especially suitable for detecting low concentrations with a high measuring sensitivity.
Optical gas detector systems, which are arranged as selected measuring stations at a plurality of measurement points in a defined area in an industrial plant or are arranged distributed in a large area, are known from the state of the art.
These optical gas detector systems with optical point detectors comprise a light source, a measuring cuvette penetrated by the light, a narrow-band optical filter and a detector for measuring the light intensity. Absorption of light by the gas to be measured leads to a reduced detector signal, which is analyzed as a measured variable.
Furthermore, gas measurement systems, with which gas concentrations of larger areas or surfaces are detected, are known from the state of the art. Such systems comprise a transmitter with a light source and with a directed receiver with detector, as well as additional optical elements for guiding the light beam. The light beam passes over distances in the range of a few m up to 50 m, 200 m or even more as an optical measured section from the transmitter to the receiver. Such a measuring unit, also called open-path measurement system, is shown in U.S. Pat. No. 6,455,854.
An arrangement with a measuring cuvette communicating with the ambient gas is described in U.S. Pat. No. 5,923,035. The measuring cuvette forms an optical measured section between a light source and a detector. The mean path traveled by the light beam in the cuvette determines the optical measured section and hence the optical path length of the measurement arrangement, the arrangement being able to be selected to be an arrangement in which the light source and the detector are arranged directly opposite each other. One mirror or a plurality of mirrors may be arranged for beam deflection or for extending the optical path length in an arrangement in which the light source and the detector are arranged on the same side for reasons of construction, or the walls of the cuvette may be made or arranged such that they reflect light, such that this leads to mirroring and/or multiple reflections in the cuvette, which extends the optical path length from the light source to the detector. Cuvettes designed in this manner are also called multi-reflection cuvettes in the state of the art. Such multi-reflection cuvettes are made completely reflecting on the inner side of the wall in the overwhelming majority of embodiments, so that part of the quantity of light emitted in an undirected manner reaches the detector over a longer path due to multiple reflections on the wall when a radiation source with undirected light beam is used. Another part of the quantity of light reaches, reflected only once, the detector over a shorter path. An arrangement for a locally limited measurement at a measurement point according to U.S. Pat. No. 5,923,035 is called a so-called point detector, for distinction from an open-path arrangement according to U.S. Pat. No. 6,455,854, because locally limited measurements can be carried out with such a measurement arrangement. An industrial area can be purposefully and specifically monitored, for example, via a spatially distributed array of a plurality of such point detectors at a plurality of measurement points. A special form of such point detectors is a mobile manual measurement device for measurement tasks that are limited in space, i.e., selected measurement tasks, with the special feature that the measurement point can be selected by the user as a variable and mobile measurement point.
The intensity of the light beam detected at the detector, which intensity can be measured by the measurement arrangement, is determined by the absorption properties of the gas or gas mixture present in the cuvette and the length of the optical path length. The presence and the concentration of certain gas components or gas species in the area of the measured section can be inferred from the spectrum detected and the signal intensity of individual wavelengths in the spectrum in a very precise manner.
Depending on the measurement task, the optical path length is determined by the structural conditions of the measurement environment. In a measurement arrangement for use in a locally limited single-point measurement for concentration measurement, the structural conditions, such as the dimensions of the housing and the length of the cuvette, preset the optical path length in a range of a few cm, for example, in a range of 3 cm to 23 cm. Besides the optical path length as twice the overall length of the cuvette in case of the use of a radiation source with undirected light beam, further optical paths are obtained statistically due to the multiple reflection in measurement arrangements with multi-reflection cuvettes, in which the light is emitted, originating from the radiation source, into the cuvette, is reflected on the opposite wall and on the lateral walls and reaches the detector arranged next to the radiation source. A mean optical path length is thus obtained, which is greater than that determined by the actual overall length. For example, in case of an overall cuvette length with an inner dimension of 5 cm, an effective optical path length of 7 cm to 50 cm is obtained, depending on the construction and the reflection properties of the multi-reflection cuvette. In an application for concentration measurement by means of an open-path measurement system in the free field, the optical path length is in the range of a few m to more than 50 m and up to 200 m or even more.
These structural conditions lead to design criteria and selection criteria for the selection of a suitable measurement wavelength M for a high-resolution and specific measurement of the concentration of a certain gas. It is desirable, in principle, for the highest sensitivity possible to be present over the entire measuring range in the presence of a concentration of the target gas in the gas mixture.
Furthermore, multireflection cuvettes are advantageous in the sense that no collimated light beam is necessary as a light source, unlike in an open-path free field arrangement according to U.S. Pat. No. 6,455,854, in which the light source (transmitter) and detector (receiver) must be exactly aligned with one another and the light beam per se is aligned, for example, by a laser light source, or additional optical components, such as lenses and/or mirrors are present for focusing, guiding and deflecting the light in case of using a light source without a limited beam collimation in the light source itself. Multireflection cuvettes are especially advantageous for practical use because, unlike in the case of a construction with mirror elements as an individual reflection point in case of contamination of the only mirror element, measurement still continues to be possible in case of partial contamination of the reflecting surfaces in the cuvette. This insensitivity to locally limited local contaminations arises from the fact that since the light is coupled from the light source into the multireflection cuvettes in the undirected form, statistically locally random and varying reflections occur on the reflecting surfaces, both as single, double and even multiple reflections, before the light reaches the detector. As a result, a locally limited local contamination does not become an effective drawback for the gas concentration measurement due to the statistically random and continuously varying reflections.
In a practical implementation with a measuring range of interest for monitoring explosion limits of a target gas, it is advantageous that, for example, the presence of a target gas in the measuring cuvette with a concentration corresponding to the lower explosion limit brings about light absorption, which causes a reduction of the signal at the detector in the range of about 10% to 15%, for example, for the target gas methane. Furthermore, the most uniform rise possible of the characteristic curve of the light absorption or of the reduction of the signal at the detector over the measuring range of interest is desirable. This leads, on the one hand, to a marked measurement effect due to the target gas, and, on the other hand, further light absorption due to cross sensitivities to other gases and the effect of environmental conditions, such as temperature fluctuations, effect of air pressure and effect of moisture, can be accepted, without the light being able to be absorbed, for example, by water of condensation almost completely at the optical path length.
To make it possible to achieve this design criterion concerning the light absorption, measurement wavelengths specifically suitable for the measured gas are to be selected in combination with suitable optical path lengths.
To make it possible to measure low concentrations with high measuring sensitivity, it is necessary to adapt the construction of the measurement arrangement and the adaptation of the components of the measurement arrangement to the measurement task and to the environmental conditions for the operation of the measurement arrangement very accurately. It is essential for high sensitivity of the measurement and for specific selectivity for certain target gases that the measurement wavelength be selected for the target gas and for the optical path length of the measured section in the cuvette. The overall size of the cuvette is preset essentially by the space available for installing the gas-measuring device, but the space available for installation has its limitations for mobile gas-measuring devices concerning applicability for mobile applications. The length of the measured section within the cuvette is limited, on the one hand, by the losses of light over the measured section itself and the light absorption by the target gas, and further essential limitations arise from the losses that occur due to the optical components and are due to the sensitivity of the detector used.
The possibility of distortions of the measurement due to environmental conditions such as cold, heat and humidity is an essential parameter affecting the precision of the measurement arrangement. The moisture in the cuvette can be reduced, in principle, in case of use in a moist measurement environment by reducing the relative humidity within the cuvette and by preventing condensation on the walls of the cuvette as well as on the light source and detector.
Heating of the optical system is a known and effective method for this according to the state of the art, as this is described, for example, in U.S. Pat. No. 7,406,854.
Such heating is not quite uncritical for using the measurement arrangement in areas with explosion hazard. To prevent an explosion of the possibly explosive gas present in the environment due to possible electric sparks or an energy discharge in the measurement arrangement, it is absolutely necessary to design the measurement arrangement such that no sparks or critical quantity of energy, which could cause ignition of a gas mixture in the measurement environment, can pass over from the measurement arrangement into the measurement environment. Furthermore, if no explosion occurs in the measurement arrangement proper, the explosion must remain limited to the measurement arrangement and must not pass over or flash over into the measurement environment. The entry of gas from the measurement environment to and into the measuring cuvette is designed in a specially secured manner for this by means of a protective element. The entry of gas is provided with a special protective element in such an explosion-proof design.
A gas sensor of an explosion-proof design is known from U.S. Pat. No. 7,285,782, wherein the gas exchange takes place via a gas exchange opening, which is provided with a dust filter consisting of a sintered material or a metal gauze and with a moisture protection filter as a protective element.
In an expanded embodiment, the gas sensor described in U.S. Pat. No. 7,285,782 has an infrared detector with at least one gas-specific measuring channel 1, 2, 3, as well as a reference channel 0, which has a zero signal, i.e., a signal that is not affected by the target gas. To take into account and compensate environmental effects, a temperature sensor, a moisture sensor and a pressure sensor are additionally provided at the infrared detector.
Despite a protective element at the gas outlet opening, heating for the measurement arrangement may be carried out with a moderate quantity of energy only in the explosion-proof design, so that, for example, a temperature rise of 5° C. to 10° C. relative to the ambient temperature is reached. A temperature rise by 10° C. in the measuring cuvette at an ambient temperature of 10° C. and a relative air humidity of 100% causes a reduction of the relative humidity of the air to about 70% to 80%, so that condensation is prevented from occurring. Any reduction in the relative humidity of the air in the measuring cuvette brings about, in principle, a reduction of the impairment of the measurement by water of condensation or droplets of water in the cuvette.
Measurement tests have revealed that measurement is affected especially in measurement arrangements for the infrared optical measurement of gases and gas mixtures in case of a design embodiment as multireflection cuvettes of an explosion-proof design with a protective element and despite heating if the measurement arrangement was exposed to a moist measurement environment during the operation before or is exposed now to such a moist measurement environment.
A solution concerning avoiding condensation for a measurement arrangement of a non-explosion-proof design can be obtained in a simple manner by markedly intensifying the heating and heating the measuring cuvette as a result to the extent that the relative humidity of the air in the measuring cuvette will be reduced to values between 5% and 10% relative air humidity. The consequence of this is always an undesired, high energy consumption for such a measurement arrangement.
Any increase in the heat output during the operation is associated, however, with the drawback for mobile devices, of both an explosion-proof design and a non-explosion-proof design, that the quantity of electric energy needed therefore must be carried along additionally in the form of an energy storage device (batteries rechargeable batteries), which would result in a disadvantageous excess weight for mobile applications, or that the available operating time becomes shorter for mobile application without increasing the weight of the device, which is not realistic in case of the application scenarios of mobile gas-measuring devices in practical use.
The explosion protection measures must be massively intensified for a measurement arrangement of an explosion-proof design, which is likewise associated again with drawbacks for the mobile use, besides a globally increased weight and volume, and additionally also has consequences for the construction and design of the protective element at the gas inlet. This protective element, made of sintered material, sintered metal or metal gauze, must be designed structurally, like the other components of the housing of the measurement arrangement as well, such that the quantity of energy made available for the heating can be retained in the measurement arrangement in case of an incident. This circumstance leads to a very massive construction for the protective element, which makes the access of gas difficult and causes disadvantageous changes in the measurement properties, and, in particular, the response characteristic in case of a gas exchange, usually characterized by the so-called t10-90 time, is adversely effected by the longer diffusion time of the gas through the protective element, which is now necessary, in such a way that the warning function of the measurement arrangement will not occur with a short delay after a change in concentration in the measurement environment. The t10-90 time is defined for a gas-measurement arrangement as the time needed for detecting, outputting and displaying a change in the gas concentration from 10% of a target gas concentration to 90% of a target gas concentration. Further difficulties arise in the embodiment for the use of the measurement arrangement in areas with explosion hazard, because the quantity of energy being carried along and the temperatures of elements in the measurement arrangements are limited by regulatory requirements.
Thus, it is not possible in a practical embodiment to prevent condensation nearly completely by means of increasing the heat output while bringing about a marked reduction of the relative humidity of the air in the measuring cuvette to values between 5% and 10% for both a measuring cuvette of non-explosion-proof design and a measuring cuvette of an explosion-proof design.
A device and a method for compensating environmental effects by means of two reference wavelengths is described for an open-pass measuring system with an optical path length of one meter to one thousand meters in U.S. Pat. No. 6,455,854. A wavelength range of 2,100 nm to 2,400 nm with one measurement wavelength and two reference wavelengths, which range is suitable for the optical path length of one meter to one thousand meters, is used here for the measurement of alkanes. The reference wavelengths are characterized in that they are not subject, in principle, to any effect of the measured gas or other gases of the measurement environment. At 2,300 nm, the measurement wavelength is selected essentially in the middle between a first reference wavelength R1 at 2,215 nm and a second reference wavelength R2 at 2,385 nm. According to U.S. Pat. No. 6,455,854, the effect of fog and rain on the absorption at the measurement wavelength is compensated by the effect at the first reference wavelength R1 with the effect at the second reference wavelength R2 having the same effect as the effect at the measurement wavelength.
The wavelengths selected in U.S. Pat. No. 6,455,854 (2,215 nm, 2,300 nm, 2,385 nm) are suitable, according to U.S. Pat. No. 6,455,854, for optical measured sections with lengths ranging from one to a thousand m. Due to this length, the absorption of the IR light by the target gas is high enough to obtain a metrological effect.
The wavelengths selected in U.S. Pat. No. 6,455,854 (2,215 nm, 2,300 nm, 2,385 mm) are thus suitable for the metrological monitoring of large areas with an open measured section (open path). The light-emitting light source (transmitter) and the light-receiving detector (receiver) or the light-receiving detectors (receivers) are arranged at great distances from each other in space (1 m<1<1,000 m), ranging from a few m to a thousand m in an open-path measuring system according to U.S. Pat. No. 6,455,854.
The wavelengths selected in U.S. Pat. No. 6,455,854 (2,215 nm, 2,300 nm, 2,385 nm) are less suitable for substantially shorter optical measured sections, as they are used in measuring devices with a closed optical measured section (1<0.3 m), in which the light-emitting light source (transmitter) and the light-receiving detector (receiver) or the light-receiving detectors (receivers) are arranged close next to each other together in a measuring cuvette as point detectors with an optical measured section shorter than 0.3 m, because the absorption of the IR light by the target gas over the optical measured section (1<0.3 m) is not high enough at these wavelengths to achieve an appreciable measurable and useful effect to detect, for example, a lower explosion limit (LEL) with the required accuracy. The consequence of this is that these wavelengths (2,215 nm, 2,300 nm, 2,385 nm) are unsuitable for a measurement arrangement designed as a point detector for the measurement of alkanes by means of a cuvette, whose effective optical path length is markedly shorter than 30 cm due to the overall size.
The combination of a moisture effect with a salt effect occurs as a special environmental effect during operation especially in a maritime environment. The ambient air additionally contains very fine salt crystals, besides the moisture, e.g., on an offshore drilling platform or onboard ships. These salt crystals enter the measuring cuvette in the form of an aerosol and are deposited as a very thin crystalline salt film on the inner walls of the measuring cuvette, as well as on the optical components, such as lenses, filters and mirrors.
This salt film has a hygroscopic action and causes more moisture to be “drawn into” the measuring cuvette continually from the measurement environment after a single-time contamination with salt-containing aerosol. The degree of relative humidity itself is determined by the intrinsic heating by the radiation source and can be reduced by an additional heating of the detector and/or of the cuvette to values in the range of 80%, so that contamination with the formation of larger drops of water on the optical components and on the inner walls of the cuvette can then be avoided. It is ensured hereby that the measurement arrangement does not become “optically blind.” The term “optically blind” means in the sense of the present invention that a reflection does not occur any more on the optical elements provided for that purpose during the operation.
Due to the hygroscopic properties of the crystalline salt film, moisture is drawn into the measuring cuvette from the measurement environment, which represents a nearly permanent operating situation with atmospheric humidity in the measuring cuvette following a single-time contamination by salt-containing ambient air. This permanent operating situation determines the measurement conditions for the measurement properties of the measurement arrangement over rather long periods of time ranging from months to years as a measurement condition with the continuous presence of atmospheric humidity in the measuring cuvette, and only the formation of condensation and droplets of water on the walls of the cuvette and the optical components, such as lenses, filters and mirrors can be prevented by heating, but the continuous presence of atmospheric humidity at variable and unknown concentrations cannot. This atmospheric humidity does affect the light absorption at the measurement wavelength and hence the possibility of metrologically detecting the target gas concentration and the metrological precision and reproducibility of such detection.
Furthermore, the ambient air frequently contains further components which can adhere with the water film to the surfaces of the cuvette, to the detectors and the optical elements in many areas of application.
Combustion residues of fossil fuels, such as smoke and soot particles, may be mentioned as such other components of the ambient air, especially in an ambient air with high atmospheric humidity, for example, fog. The effect of these environmental effects, especially of the continual, further supply of atmospheric humidity into the cuvette by the crystalline hygroscopic salt film, cannot be compensated by heating alone, especially for a measurement arrangement of an explosion-proof design in scenarios of use such as offshore drilling platforms or onboard ships.
The continual penetration of atmospheric humidity into the cuvette due to the hygroscopic effect of the fine crystalline salt films cannot be fully compensated even for a measurement arrangement of a non-explosion-proof design by changing the heating of the detector and/or cuvette, e.g., by increasing the heat output or by raising the heating temperature or even by a cyclically performed heating of the entire measurement arrangement.
These environmental effects likewise cannot be fully compensated either by the use of additional sensors, such as temperature, moisture and pressure sensors in the cuvette or at the detectors.